1. Field of the Invention
The present invention relates to a control system for an internal combustion engine, and more particularly to a control system for an internal combustion engine for realizing quick activation of a catalyst converter when the internal combustion engine is cold.
2. Description of the Related Art
In is recently required for engine emission control to maintain a catalyst converter in an exhaust pipe of an internal combustion engine always at an adequate active state. For instance, it is desired to shorten engine warm-up time and to activate the catalyst converter quickly in warming up the catalyst converter from the cold state in starting an engine at low temperature.
It is required for engine emission control recently to maintain a catalyst converter provided in an exhaust pipe of an internal combustion engine always at an adequate active state. For instance, it is desired to shorten an engine warm-up time and to activate the catalyst converter quickly in warming up the catalyst converter from the cold state in starting an engine at low temperature.
In JP-A-61-232317, it is proposed to advance timing of opening an exhaust valve of an internal combustion engine to supply exhaust gas having a large amount of heat energy to an exhaust pipe to raise exhaust gas temperature. In JP-A-61-190118, it is proposed to advance valve timing of an exhaust valve when an internal combustion engine is cold to reduce combustion gas expansion ratio and to raise exhaust gas temperature.
However, those conventional systems are directed to raise the temperature of the exhaust gas just by thermal energy of combustion gas within a cylinder. The effect of raising the temperature does not necessarily result in activating the catalyst quickly. Although a start catalyst whose capacity is relatively small has been put into practical use in order to activate the catalyst quickly, such a quick activating type catalyst has been also required to be able to activate more quickly.
The present invention has an object to provide a control system for an internal combustion engine which can activate the catalyst converter quickly when the internal combustion engine is cold.
According to the present invention, a variable valve timing mechanism is controlled when an internal combustion engine is cold to control valve opening overlap between an intake valve and an exhaust valve to a predetermined amount.
Burned gas within a cylinder is blown back to an intake port side and flows into the cylinder again, i.e., internal EGR is actively conducted, so that combustion velocity of fuel within the cylinder becomes relatively slow by increasing the valve opening overlap of the intake valve and the exhaust valve. At this time, unburned fuel not burned within the cylinder is emitted to an exhaust pipe to be burned by itself. Here, the phenomenon in which the unburned fuel is burned within the exhaust pipe at the upstream of the catalyst converter will be defined as post-burning. Temperature of exhaust gas may be kept high by thus actively conducting post-burning within the exhaust pipe. Then, the catalyst converter in the cold state may be activated quickly by feeding the high temperature exhaust gas to the catalyst converter. As a result, the catalyst converter may be activated quickly when the internal combustion engine is cold.
Here, because the temperature of exhaust gas emitted via the exhaust valve after combustion within the cylinder reaches about 700xc2x0 C. at which gasoline fuel fires by itself, most of the unburned fuel (unburned HC) is burned during the post-burning. Accordingly, the amount of HC as the unburned fuel (unburned HC) fed to the catalyst converter before it is activated is very small.
Preferably, the valve opening overlap of the intake valve and the exhaust valve is set around 20 to 30xc2x0 crank angle rotation (CA). Concentration of HC of unburned gas emitted out of the cylinder increases and the effect of the above internal EGR may be attained reliably by setting the valve opening overlap within the above range.
Preferably, ignition timing of the internal combustion engine is also controlled to the retard side when the internal combustion engine is cold. Fuel (air and fuel mixture) flowing in the cylinder starts to burn slowly and unburned fuel is emitted to the exhaust pipe when the exhaust valve is opened to be burned in the post-burning by retarding the ignition timing.
Preferably, an air-fuel ratio of intake air and fuel mixture is controlled to a stoichiometric air-fuel ratio or a slightly lean ratio when the internal combustion engine is cold. The amount of oxygen within the exhaust gas increases and the post-burning may be carried out more reliably by controlling the air-fuel ratio to the stoichiometric air-fuel ratio or to the slightly lean ratio. In this case, the effect of increasing the temperature of exhaust gas may be attained further by controlling the air-fuel ratio to the stoichiometric air-fuel ratio or the slightly lean ratio while controlling the valve opening overlap and the ignition timing.
Preferably, the variable valve timing mechanism is controlled when the internal combustion engine is cold to vary valve opening timing of the exhaust valve to the advance side more than that right after starting the engine. The remaining unburned fuel is emitted quickly via the exhaust valve by advancing the opening timing of the exhaust valve. Accordingly, it becomes possible to emit the exhaust gas around the peak of the cylinder inner temperature, thus contributing to the promotion of the post-burning. In this case, the effect of increasing the temperature of the exhaust gas may be attained further by advancing the valve timing of the exhaust valve while controlling the valve opening overlap, the ignition timing and the air-fuel ratio.